One of the key components of a computer system is a place to store data. Typically computer systems employ a number of storage means to store data for use by a typical computer system. One of the places where a computer can store data is in a disk drive which is also called a direct access storage device.
A disk drive or direct access storage device includes several disks which look similar to 45 rpm records used on a record player or compact disks which are used in a CD player. The disks are stacked on a spindle, much like several 45 rpm records awaiting to be played. In a disk drive, however, the disks are mounted to spindle and spaced apart so that the separate disks do not touch each other.
The surface of each disk is uniform in appearance. However, in actuality, each of the surfaces is divided into portions where data is stored. There are a number of tracks situated in concentric circles like rings on a tree. Compact disks have tracks as do the disks in a disk drive. The tracks in either the disk drive or the compact disk essentially replace the grooves in a 45 rpm record. Each track in a disk drive is further subdivided into a number of sectors which is essentially just one piece of the track.
Disks in a disk drive are made of a variety of materials. Most commonly, the disks used in rotating magnetic systems is made of a substrate of metal, ceramic, glass or plastic with a very thin magnetizable layer on either side of the substrate. Such a disk is used in magnetic, and magneto-optical storage. Storage of data on a such a disk entails magnetizing portions of the disk in a pattern which represents the data. Other disks, such as those used in CD's, are plastic. Data, such as songs, is stored using a laser to place pits in the media. A laser is used to read the data from the disk.
As mentioned above, to store data on a disk used in a rotating magnetic system, the disk is magnetized. In order to magnetize the surface of a disk, a small ceramic block known as a slider which contains at least one magnetic transducer known as a read/write head is passed over the surface of the disk. Some ceramic blocks contain a separate read head and a separate write head. The separate read head can be a magnetoresistive head which is also known as an MR head. The ceramic block is flown at a height of approximately six millionths of an inch or less from the surface of the disk and is flown over the track as the transducing head is energized to various states causing the track below to be magnetized to represent the data to be stored. Some systems now also use near contact recording where the slider essentially rides on a layer of liquid lubricant which is on the surface of the disk. With near contact recording, the ceramic block passes even closer to the disk.
To retrieve data stored on a magnetic disk, the ceramic block or slider containing the transducing head is passed over the disk. The magnetized portions of the disk generate a signal in the transducer or read head. By looking at output from the transducer or read head, the data can be reconstructed and then used by the computer system.
Like a record, both sides of a disk are generally used to store data or other information necessary for the operation of the disk drive. Since the disks are held in a stack and are spaced apart from one another, both the top and the bottom surface of each disk in the stack of disks has its own slider and transducing head. This arrangement is comparable to having a stereo that could be ready to play both sides of a record at anytime. Each side would have a stylus which played the particular side of the record.
Disk drives also have something that compares to the tone arm of a stereo record player. The tone arm of a disk drive, termed an actuator arm, holds all the sliders and their associated transducing heads, one head for each surface of each disk supported in a structure that looks like a comb at one end. The structure is also commonly called an E block. A portion of metal, known as a suspension, connects the sliders to the E block. At the other end of the actuator is a coil which makes up a portion of an voice coil motor used to move the actuator. The entire assembly is commonly referred to as an actuator assembly.
Like a tone arm, the actuator arms rotate so that the transducers within the sliders, which are attached to the actuator arm can be moved to locations over various tracks on the disk. In this way, the transducing heads can be used to magnetize the surface of the disk in a pattern representing the data at one of several track locations or used to detect the magnetized pattern on one of the tracks of a disk. Actuators such as the ones described above are common to any type of disk drive whether its magnetic, magneto-optical or optical.
One of the most critical times during the operation of a disk drive is just before the disk drive shuts down. When shutting down a disk drive, several steps are taken to help insure that the data on the disk is preserved. In general, the actuator assembly is moved so that the transducers do not land on the portion of the disk that contains data. How this is actually accomplished depends on the design of the drive. The disk drive design of interest for this invention includes a ramp. Disk drives with ramps are well known in the art. U.S. Pat. No. 4,933,785 issued to Morehouse et al. is one such design. Other disk drive designs having ramps therein are shown in U.S. Pat. Nos. 5,235,482 and 5,034,837.
Typically, most of the ramp is situated off to the side of the disk. A portion of the ramp is positioned over the disk itself. In operation, before power is actually shut off, the actuator assembly swings the suspension or another portion of the actuator assembly up the ramp to a park position at the top of the ramp. This is much like a child running up a playground slide backwards and sitting at the top of the slide. When the actuator assembly is moved to a position where parts of the assembly are at the top of the ramp, the sliders or ceramic blocks, which include the transducers, are positioned so that they do not contact the disk. Commonly, this procedure is known as unloading the heads. Unloading the heads helps to insure that data on the disk is preserved since, at times, unwanted contact between the slider and the disk results in data loss on the disk.
Startup of a disk drive with a ramp is an even more critical time. Startup includes moving the actuator assembly so that the suspension slides down the ramp and so that the slider flies when it gets to the bottom of the ramp. This is much like a water slide where the bottom of the pool is the disk and the slide is the ramp. When the rider gets to the bottom, he "flies" by skimming across the water rather than touching the bottom of the pool. In other words, the best ramp control designs prevent contact between the slider and the disk so as to prevent any type of data loss.
The most common mechanical design which assures that the slider will fly requires a ramp with a very gentle slope. There are problems associated with this design. Most importantly, a gentle sloping ramp is longer than a short ramp and requires more space. Space is becoming more precious as the form factor of the disk drive shrinks. Currently, the smallest disk drive on the market has a disk with a diameter of 1.3". Also on the market are PCMCIA form factor disk drives. The PCMCIA disk drives have the length and width of a credit card. The height of these drives varies. The disk in such a drive has a diameter of about 1.8". Packing a long ramp in such a small packages is difficult. Even if it can be done, them will be a push toward steeper ramps since with a steeper sloping ramp, more of the disk surface can be devoted to storing data to satisfy the consumer's thirst for increased data capacity.
A way to accommodate a steeper ramp is to control the velocity of the slider as it moves down the ramp. If the velocity can be controlled, the downward portion of the speed can be controlled so that the slider will not result in the slider hitting the disk. U.S. Pat. No. 4,864,437 issued to Couse et al. teaches one way of controlling the velocity of the slider as it moves down a ramp. In Couse et al., the voltage across a voice coil motor is monitored and controlled. The voltage across the voice coil motor includes a small component of the total voltage known as Back EMF. A voice coil motor includes magnets and an actuator coil. When the actuator coil cuts a magnetic field, Back EMF is generated. The Back EMF varies as a function of the velocity of the actuator coil through the magnetic field produced by the magnets of the voice coil motor and, presumably, as a function of the velocity of the actuator down the ramp. Thus, it is possible to get an estimate of the rotational velocity of the actuator from the Back EMF of the actuator motor. From the rotational velocity estimate and knowing the designed slope of the ramp one can calculate the component of velocity in the vertical direction (perpendicular to the surface of the disk). It is important to carefully control the vertical velocity in order to prevent any damage to the disk surface.
The design of the velocity control in Couse et al. also has problems. Most importantly, the Back EMF is a very small component of the total voltage across the coil of the actuator. This component will also become smaller as additional current is passed through the coil. The Back EMF signal is also prone to noise. In short, since the Back EMF component of the voltage across the actuator is small and prone to noise, it does not always reliably reflect the actual velocity of the slider. In addition, as the operating temperature of the disk drive increases, the noise level increases making the Back EMF an even smaller component and even more prone to noise. If there happens to be an error indicating that the velocity is slower than it actually is, then an increase in the actuator coil current may cause the velocity of the slider down the ramp to increase to the point where the slider will contact the surface of the disk. This could cause a head crash resulting in a loss of data. It should be remembered that contact between the disk and the slider might not instantly cause data loss. Many times it causes particle generation within the disk enclosure. Generated particles, although seemingly small in everyday terms, are "boulders" to a slider that is flying at less than six millionths of an inch from the surface of the disk.
Another problem is that at the smaller form factors the torque constant of the actuator moves down drastically which means that the actuator motor can not get the slider moving as quickly in a smaller form factor drive. For example, the torque constant in the current 1.8" PCMCIA type drives is approximately 10-15% of the torque constant in a 2.5" drive (the next larger form factor). In essence, smaller form factor drives must use smaller actuator motors which produce less torque. This problem also gets worse as the height dimension of the drive shrinks since a smaller actuator motor is used. Thus, as the actuator motors get smaller there is less torque to get the actuator moving and therefore less Back EMF signal produced over the short amount of stroke for the sliders to move down the ramp. The result is that the noise further drowns out the smaller Back EMF signal produced by the slower moving actuator.
Yet another problem with using the Back EMF of the actuator motor to determine speed is that the Back EMF varies with the temperature of the permanent magnets and the coil resistance in the voice coil motor. In small form factor drives that may go into a laptop or sub-laptop computer there may be instances where the drive may park the sliders on the ramp and, within minutes, slide them back down the ramp again. In these applications, the disk drive will be at or near the operating temperature of the drives. The operating temperature of a drive can be up to 50 degrees Celsius higher than the drive when it first starts from room temperature. The Back EMF can vary as much as 10-15% over such a temperature range. Of course this amount of difference in the Back EMF signal translates into a 10-15% difference in the vertical component of velocity of the transducer which may well result in the head contacting the disk.
Of course, the system can be designed to accommodate "worst case" situations but this results in a sub-optimal design.
In addition, the prior art teaches no way of estimating what the initial values of several data points might be so that the velocity function can be accurately estimated while the transducer is on the ramp.
Although the measure of Back EMF is a closed loop process in terms of velocity control, the use of Back EMF does not indicate position. There is a problem, potentially, in not knowing the position of the transducer as it moves over the ramp.
Thus, what is needed is a device that can accurately and repeatably determine the velocity of the slider as it moves down the ramp without regard to temperature fluctuations, differing noise levels, or changes in the temperature of the actuator motor. In addition, what is needed is a device that allows an estimate of the velocity to be made from a constant generated while the transducer is still parked on the ramp making a single measurement. Preferably, the velocity of the slider will not be determined based on the Back EMF of the actuator coil of the actuator assembly.